When unRAID rebuilds a disk, it reads parity and all the other disks and uses this data to calculate the data of the missing disk.
When you read an emulated disk, it does the exact same thing, calculating the missing data from parity and all the other disks.
When you write an emulated disk, it reads parity and all the other disks, calculates the parity change that would happen due to the write, and writes parity. So even writing when a disk is missing or being rebuilt, still results in data that can be calculated by reading parity and all the other disks.
Have you seen the wiki about how parity works? It isn't very complicated and unRAID makes a lot more sense if you understand parity.
https://wiki.lime-technology.com/UnRAID_6/Overview#Parity-Protected_Array